#back-to-top {
    position: fixed;
    bottom: 2rem;
    right: 2rem;

    background: #ccc;
    color: #000;
    border-radius: 50%;
    width: 48px;
    height: 48px;
    line-height: 48px;
    text-align: center;
    font-size: 24px;
    cursor: pointer;

    box-shadow: 0 2px 6px rgba(0,0,0,0.25);
    transition: opacity 0.2s ease;
    z-index: 9999;
}